This invention relates to feedback control of the recirculation of exhaust gas through an internal combustion engine, particularly an automotive engine.
Concerning the prevention of air pollution attributable to exhaust gas of internal combustion engines, particularly of atomotive use, the recirculation of a portion of exhaust gas back into the engine intake is a widely employed technique for suppressing the emission of NOx. The recirculation of exhaust gas (EGR) has the effect of lowering the maximum combustion temperature in the engine combustion chambers so that the formation of NOx can be suppressed.
The suppressive effect of EGR on the formation of Nox is enhanced as the volume of the recirculated exhaust gas relative to the volume of fresh air admitted into the engine is increased (this volume ratio will herein be referred to as EGR rate). To maintain NOx emission below a permissible level, there is a need of effecting EGR at considerably high EGR rates. On the other hand, the employment of high EGR rates tend to cause instability of the engine operation. Accordingly the EGR rate must be controlled in dependence on the operating condition of the engine and high precision is required of the control especially when high EGR rates are involved in the scope of the control.
In conventional EGR control systems, it is a usual way of operating an EGR control valve to use a vacuum-operated actuator which is connected to the induction passage of the engine such that the EGR control valve is operated in dependence on the magnitude of vacuum produced either at a venturi section of the induction passage or in the neighborhood of a main throttle valve. In this type of EGR control systems, the control is accomplished in a programmed manner so as to regulate the EGR rate to a target value which is preset based on an assumed relationship between the EGR rate or the aforementioned vacuum and the condition of combustion in the engine. The magnitude of a carburetor venturi vacuum, for example, is of course an indication of the volume flow rate of air in the induction passage, but there is a limitation to the precision in the control of EGR when the control valve is operated merely on the basis of, for example, the venturi section vacuum. It is inevitable that a considerable fluctuation occurs in an actualized EGR rate with changes in the engine operating condition, possibly causing instability of the engine operation, increase in fuel consumption and/or failure in maintaining a satisfactorily low level of NOx emission, but such fluctuation cannot be corrected insofar as the actualized EGR rate is not detected.